One of the routine steps in the production process of integrated circuits is the inspection of patterned surfaces, especially when starting up a new design. A substantial part of the whole 300 mm wafer is imaged to check for defects in the pattern and for particles imbedded in the pattern or on top of the wafer. This kind of inspection is presently performed by high-throughput optical microscopy in dedicated instruments.
With the progress in lithography, the instruments must detect smaller and smaller defects and particles. A problem is that the light scattering from particles rapidly decreases when the particle's size decreases, so the signal-to-background (and noise) ratio is decreasing.
In order to solve this problem, electron beam inspection machines have been used and for some purposes are still in use. Electron beam inspection machines can have a much higher resolution than optical system. However, electron beam inspection machines are limited in the speed at which the electron beam inspection machines can inspect a wafer. In order to increase the speed, multi-beam electron beam systems have been proposed.
U.S. Pat. No. 6,774,646 describes a method and an apparatus for inspecting samples. The method uses an electron beam inspection system using multiple electron beams. The apparatus comprises an electro-magnet to establish a magnetic focusing field which is generated in the direction of the axis of the electron beams. The magnetic field is created by using two pole-pieces, one located above the electron emitters and one located under the stage which carries the sample. The method further uses an electrostatic deflection field, which is generated in a direction perpendicular to the magnetic field by using two plates positioned on opposite sides of the electron beam trajectory. The deflection force produced by the combined effect of the electrical and magnetic fields (E×B) is a deflection in the lateral direction, which is deflection is also used to disentangle the beam path of the primary electrons which are directed towards the sample, and the beam path of the secondary electrons which are directed towards the detector array.